IP Multimedia (IPMM) services provide a dynamic combination of voice, video, messaging, data, etc, within the same session. By growing the numbers of basic applications and the media which it is possible to combine, the number of services offered to the end users will grow, and the inter-personal communication experience will be enriched. This will lead to a new generation of personalised, rich multimedia communication services, including so-called “combinational IP Multimedia” services.
IP Multimedia Subsystem (IMS) is the technology defined by the Third Generation Partnership Project (3GPP) to provide IP Multimedia services over mobile communication networks. IMS provides key features to enrich the end-user person-to-person communication experience through the integration and interaction of services. IMS allows new rich person-to-person (client-to-client) as well as person-to-content (client-to-server) communications over an IP-based network. The IMS makes use of the Session Initiation Protocol (SIP) to set up and control calls or sessions between user terminals (or user terminals and application servers). The Session Description Protocol (SDP), carried by SIP signalling, is used to describe and negotiate the media components of the session. Whilst SIP was created as a user-to-user protocol, IMS allows operators and service providers to control user access to services and to charge users accordingly. Other protocols are used for media transmission and control, such as Real-time Transport Protocol and Real-time Transport Control Protocol (RTP/RTCP).
Within an IMS network, Call Session Control Functions (CSCFs) perform processing and routing of signalling. CSCFs handle session establishment, modification and release of IP multimedia sessions using the SIP/SDP protocol suite. 3GPP TS23.228 describes the logical nodes P-CSCF, I-CSCF, S-CSCF, E-CSCF and BGCF. The S-CSCF conforms to 3GPP TS 24.229 and performs session control services for User Equipments (UEs). It maintains session state to support the services, and performs the following functions:                It acts as a registrar according to [RFC3261] at registration;        It notifies subscribers about registration changes;        It provides session control for the registered users' sessions;        It handles SIP requests, and either services these internally or forwards them on to a further node; and        It interacts with IMS Application Servers.        
The S-CSCF performs SIP routing according to 3GPP routing procedures. For inbound SIP traffic, the S-CSCF will route sessions to that P-CSCF whose address was stored during subscriber registration. For outbound SIP traffic, the S-CSCF interrogates a DNS/ENUM to determine how the call should be routed. The S-CSCF interacts with the Home Subscriber Server (HSS) to obtain subscriber data and to exchange authentication information using DIAMETER messages.
The IMS allows IMS subscribers to initiate sessions towards non-IMS users, including users connected to conventional telephone networks. IMS subscribers may also be allocated addresses such as telephone numbers to allow for incoming calls to be made to these subscribers via external networks. This will result in increased usage of new multimedia services and therefore higher revenues for operators. The Breakout Gateway Control Function (BGCF) within an IMS network is used to select an outgoing gateway (by identifying a SIP URI and resolving that URI into an IP address) through which to route a SIP request addressed to an “external” destination address. The BGCF is specified in 3GPP TS23.228 and TS24.22. The outgoing gateway that is selected by the BGCF may be a Media Gateway Control Function, MGCF, which controls one or more Media Gateway lying in the media plane. FIG. 1 illustrates schematically a part of a network including a BGCF. Routing at the BGCF is based on, for example, calling party number, called party number, and configurable information in the BGCF. In addition, the selection of an external route can also be dependent on the routing number and/or Carrier Identification Code (CIC), or on other parameters if these other parameters are forwarded by the CSCF to the BGCF.
The BGCF may be collocated with a CSCF and invoked internally (or via an external interface) from the originating S-CSCF when there is no entry in the ENUM database for the destination telephone number and it is not otherwise possible to find an IP address for the telephone number. The BGCF is also invoked internally from the terminating I-CSCF when a location query against the HSS fails. The BGCF is also invoked internally from the E-CSCF when the target is a telephone number.
IMS standards further define an Interconnection Border Control Function (IBCF). The IBCF is used as gateway to communicate with other external networks. The IBCF handles border control for the SIP signaling interface, topology hiding and interworking with non-IMS networks. Typical functions are NAT and Firewall functions. The IBCF is in practice a Session Border Controller specialised for the NNI (Network To Network Interface).
According to existing and currently proposed IMS architectures there is no easy way to configure a BGCF or IBCF to perform different routing behaviour for specific users. If a specific user experiences a poor quality service and calls the network operator to complain, there is no easy way for the operator to control the behaviour for this specific user in the routing function. The routing tables are large and complex and the operator is likely to be unwilling to modify them for the sake of one individual user. Within the IMS signalling plane, routing of SIP signalling is similarly carried out according to block rules installed in signalling nodes such as the CSCFs. There is limited flexibility to implement routing procedures on a per user basis.